1. Field of the Invention
This invention resides in the field of immunoassays performed in a heterogeneous format with a polymeric solid phase. In particular, this invention addresses the problem of non-specific binding to the solid phase in serum assays and the false positive readings that are caused by such binding.
2. Description of the Prior Art
Human serum is frequently analyzed by immunoassay as a highly effective means of achieving a selective determination of serum analytes for purposes of both diagnosis and monitoring. Typical analytes determined by immunoassay include biochemical markers which when either elevated or depleted relative to their normal levels serve as indications of an abnormal physiological condition or a predilection toward disease. Other analytes include foreign substances or antibodies raised against foreign substances, either of which serves as an indication of exposure. In each case, the antigen-antibody interaction at the core of the assay provides the assay with its specificity, and the coupling of one of the binding members to a solid phase provides the assay with a highly convenient means of separating bound from unbound species.
The solid phase can assume a variety of forms and configurations, ranging from the internal walls of wells in a microtiter plate to microspheres, latex particles, or beads. A disclosure of one of the more sophisticated uses of microspheres is found in Watkins et al., U.S. Pat. No. 6,280,618 B2, dated Aug. 28, 2001, “Multiplex Flow Assays, Particularly With Magnetic Particles as Solid Phase,” in which the microspheres are both magnetic and classifiable to permit a multitude of assays to be performed simultaneously on a single sample and all phase separation steps to be performed quickly and cleanly on a micro-scale with detection by flow cytometry. A disclosure of a particular multi-analyte immunoassay that can be performed in this manner is found in Bio-Rad Laboratories, Inc., International Patent Application No. WO01/79844, entitled “Multi-Analyte Diagnostic Test For Thyroid Disorders,” published Oct. 25, 2001, under the Patent Cooperation Treaty. The contents of both U.S. Pat. No. 6,280,618 B2 and International Patent Application No. WO01/79844 are incorporated herein by reference in their entirety.
Certain serum samples demonstrate across-the-board positivity, i.e., a binding of IgG from the sample to the solid phase independently of the presence or absence of the analyte. This can occur both in assays in which the analyte is itself an antibody with a particular binding specificity and those in which the analyte is an antigen whose binding to the solid phase is detected by a second antibody binding. The binding of IgG, whether it be a component of the assay or extraneous serum IgG binding in a non-specific manner to the solid phase, is typically detected by labeled anti-human IgG. The non-specific IgG binding can therefore increase the reading of a positive test result, and in assays for IgG's of particular specificities, the non-specific binding can produce a positive reading when the analyte is absent, either case providing a misleading assay result.